1. Field of the Invention
The present invention relates to a sensing circuit for a micro-electro-mechanical system (MEMS) microphone. More particularly, the present invention relates to an integrated alternative sensing circuit for an MEMS microphone and a sensing method thereof.
2. Description of Related Art
Micro-Electro-Mechanical System (MEMS) technique is a design based on microminiaturized mechanical structures, which among others, is mainly used in three fields including micro sensor, micro actuator, and micro structure components. The micro sensor can employ relative semiconductor process techniques, and thus can be integrated with integrated circuits (ICs). Therefore, the competitiveness of this technique is improved, and this technique is also highly regarded. Micro sensor is a micro component having characteristics of a sensor, and can converts external physical or chemical status (e.g. light, heat, magnetism, sound, pressure, position etc.) into the electrical signal for the process of any signal which usually is the signal that can be easily controlled and processed, such as voltage or current. The micro sensor using the MEMS process can have the function of conventional sensing components, and even can have the sensing function that cannot be achieved by the conventional sensing components by the use of microminiaturized micro sensing components.
Currently, many micro sensors are fabricated by the MEMS process, and for example, pressure sensors, accelerometers, IR sensors, temperature sensors, chemical sensors, flow sensors, and acoustic sensors are embodied one after another.
The emergence of MEMS microphone components impels the development of many new forms of applications. Due to the characteristics of being microminiaturized and easily integrated with IC chip for the process of signal, this MEMS microphone enables people to sense various sounds. For example, the microphones in array can determine the direction of a sound source. For example, the multi-sensor can enhance the use function of the sensing mechanism.
The MEMS microphones currently in use can be divided into two kinds, namely MEMS electret condenser microphone (ECM) and MEMS condenser microphone. The architecture of the MEMS electret condenser microphone includes a material layer, e.g., Teflon implanted into the ECM, and as the layer material has the function of accumulating charges, this kind of microphone can directly sense the change of the acoustic pressure in the absence of an applied bias, and further convert it into electrical signals for the subsequent signal processing.
The MEMS condenser microphone is a kind of microphone that does not have an electret material. In other words, when this kind of microphone is used, an applied bias, usually a voltage above 12 V, is required. Therefore, this kind of microphone when used in a subsequent circuit will lead to the increase of the overall power consumption of the chip. However, as this kind of the architecture has preferred sensing sensitivity and low sensitivity to temperature, it becomes a main objective of research.
In FIG. 1, a sensing circuit 100 of an MEMS condenser microphone according to the conventional art is shown. When the circuit is in an initial state, the MEMS condenser microphone component 110 provides a required bias at an end point N1 by the use of a bias resistor 120 through the power supply VDD, and another end of the MEMS condenser microphone component 110 is connected to a ground end GND. The bias resistor 120 and the MEMS condenser microphone component 110 form a filer for blocking unnecessary noise signals and providing signals of the frequency band for an audio. When an acoustic pressure is transmitted to the MEMS condenser microphone component 110, the displacement of the condenser changes, so the charge accumulated on the condenser changes, and further the signal changes. The signal is input to the input end of the front-end buffer amplifier 140 through the DC blocking condenser 130, such that the signal is amplified and then transmitted to the output end Vout, thus finishing the capture of signal.
In FIG. 2, a sensing circuit 200 for an MEMS electret condenser microphone according to the conventional art is shown. In the sensing circuit 200, an MEMS electret condenser microphone component 210 is an component with a built-in charge-accumulating layer, which can have the function of accumulating charges in the absence of the applied bias. The MEMS electret condenser microphone component 210 is directly connected to an end of the DC blocking condenser 230 via an end point N2, and another end is connected to the ground end GND via the end point N2. By adding a resistor 220 between the end points N1 and N2, a filter can be formed for blocking unnecessary noise signals and providing signals of the frequency band for an audio. The material of the charge-accumulating layer in the MEMS electret condenser microphone component 210 is mostly Teflon. When the acoustic pressure is transmitted to the MEMS electret condenser microphone component 210, the accumulated charge amount changes, thus changing the magnitude of the signal. The signal is transmitted to the input end of the front-end buffer amplifier 240 through the DC blocking condenser 230, and then the signal is transmitted to the output end Vout.
However, in the current application of MEMS microphones, microphones with different specifications must be selected according to different application environment. For example, in a lower power environment, the MEMS electret condenser microphones are mostly used, and in a high sensitivity environment, the MEMS condenser microphones are mostly used. The operating methods of the two kinds of microphones are not exactly the same, so a signal sensing circuit that can simultaneously process signals of the two kinds of microphones is desired, which can increase the flexibility of using the MEMS microphone and improve the service efficiency of the microphone.